Conventionally, secondary batteries have been widely used, in portable personal computers, digital cameras, camcorders, portable telephones and other electronic equipment, in electric cars, hybrid cars and other vehicles, in hybrid elevators, in power systems which combine solar cells and power generation devices with secondary batteries, in uninterruptible power supplies and other battery-mounted devices and systems, and in various other systems and devices.
And, in order to enhance convenience of use of electronic equipment such as for example portable personal computers, the remaining charge in secondary batteries available for use is displayed, or an alarm is sounded before the battery is exhausted, and similar.
In systems and devices such as solar cell power generation systems, hybrid cars and other systems and devices, from the standpoint of stable supply of power to a load, it has been necessary to ensure a state in which a secondary battery is continuously being charged to a certain extent. On the other hand, due to the need to absorb excess generated power and regenerated power, charging is controlled such that the SOC (State of Charge), which is the percentage of charged electrical quantity relative to the full charge capacity (FCC), remains in the range of, for example, 20% to 80%.
In order to detect the remaining charge quantity of a secondary battery available for use (remaining battery quantity) and calculate the SOC, the full charge capacity of the secondary battery must be known. However, the full charge capacity of a secondary battery decreases with degradation of the secondary battery, and if the full charge capacity value at the time of shipment is used without modification to determine the remaining battery quantity and SOC, the error in calculating the remaining battery quantity and SOC increases.
Methods are known in which, after a secondary battery has been shipped and usage of the secondary battery has begun, the secondary battery is completely discharged from the full-charged state, and by integrating the discharge current at this time, the full charge capacity is calculated, and the full charge capacity value is corrected or updated. However, in such methods, in order to correct or otherwise processing the full charge capacity value, it is necessary that the secondary battery in a usage state in the device or system be completely discharged from the fully charged state, and so opportunities to perform correction or updating of the full charge capacity are limited.
Methods are known (see for example Patent Document 1) in which, even when a secondary battery is not completely discharged, the charging current from the beginning of charging until the secondary battery is fully charged is integrated, and the integrated value is added to the charged electrical quantity at the time charging was begun, to calculate the full charge capacity value. By this means, even when the secondary battery is not completely discharged, the full charge capacity value can be corrected.
However, in the method described in Patent Document 1, if the secondary battery is not fully charged, the full charge capacity value cannot be corrected. Hence there is the drawback that, in cases where a user stops charging before full charging occurs, or controls charging such that the secondary battery is not fully charged as in the above-described systems and devices, the full charge capacity value cannot be corrected.    Patent Document 1: Japanese Patent Application Laid-open No. 2006-177764